Preheat the oven to 350°F. In a large bowl, beat the butter, oil and sugar on high speed with an electric mixer for 2-3 minutes, until thick and pale yellow. Beat in the eggs and vanilla.

In a smaller bowl, stir together the flour, cocoa, baking powder, baking soda and salt. Add a third to the butter mixture and beat on low or stir just until blended; add half the yogurt and half the zucchini and do the same. Repeat with another third of the flour mixture, the rest of the yogurt and zucchini, and the rest of the flour. Spoon the thick batter into two loaf pans, smoothing the top.

Bake for 30-40 minutes, until the top is cracked and springy to the touch, or still sunken in the middle - it's up to you. Cool in their pans on a wire rack.